9 6 



SYSTEMATIC ZOOLOGY 



large, though some have been found of twice that size, 

 occurring" in the tide pools are small. 



Those 



FlG. 86. Astrophyton aborescem, basket fish. (After Ludwig, from Parker and Haswell's 



Text-book.) 



CLASS IV. ECHINOIDEA 



The Echinoidea (Gr. kylvos, sea urchin, and ea$o?, form) 

 include the sea urchins, sand dollars and similar animals. The 

 common sea urchins | Fig. 87) found in the tide pools along the 

 shore are dome-shaped and covered with longer or shorter 

 spines, which are movable to a certain extent. Stalked pedicel- 

 lariae are present, similar to those which occur in the starfishes, 

 but they have three or four jaws instead of two. Projecting 

 between the spines on certain parts of the body are tube feet 

 like those in the ambulacra] grooves of the starfishes, but 

 usually more extensible. On the oral side of the animal, the 

 spines and the hard shell, or test, beneath them cease at some 



